Compressors are thermal fluid flow machines and are used for compressing gases, in particular air. Compressors find extensive application in engine construction for internal combustion engines operating on a continuous or intermittent basis and are used to compress the air required for combustion for example in reciprocating piston engines for increasing power, in gas turbines for generating electrical energy or in reaction engines for driving aircraft. The driving of the compressor is carried out for example by utilising the energy contained in the exhaust gas, but it may also be carried out in a mechanical or an electrical manner.
According to the field in which it is used, for example in a reaction engine, the compressor may be designed as an axial-flow compressor in order to achieve high mass flow rates. “Mass flow rate” shall be understood to mean an air mass which is conveyed by the compressor over a specific period of time. Alternatively, variables relating to geometric or environmental conditions, such as throughput or volumetric flow rate, may also be used to characterise the operation of the compressor. The air to be compressed flows axially against the compressor from the surroundings and is conveyed by the compressor in the reaction engine and thereby compressed. For this purpose, the compressor is generally composed of an impeller comprising compressor blades which is mounted on a shaft, which impeller rotates in a housing comprising corresponding guide blades and thus forms a compressor stage. The compressor blades, each provided with a blade base, are fitted to the impeller with play, in such a way that, in the event of a sufficiently fast rotation of the impeller on account of the occurrence of an outwardly directed centrifugal force, the compressor blades centre themselves and are embedded in the impeller. Alternatively, the blades are rigidly connected to the impeller. The guide blades are rigidly arranged at the housing. To increase the compression, a plurality of compressor stages can be arranged one behind the other in the compressor for reaction engines, thereby forming a multistage compressor. Furthermore, a fan and a second compressor can be connected upstream of the compressor. The impeller is driven by a shaft that is driven by a turbine at the end of the reaction engine.
When operating the compressor, the compressor power is set by the rotational speed of the impeller and by the mass flow rate in the compressor. For this purpose, for example the driving power of the shaft can be altered by the turbine, in order to set the rotational speed of the impeller. The mass flow rate in the compressor can be varied by means of adjustable guide blades or blow-off valves or by altering the blade tip clearance. In this manner, it is possible to set an operating point for the compressor, which operating point is defined for example by a pressure ratio and a mass flow rate, by compressor power and rotational speed or other alternatives.
The maximum pressure ratio of a compressor stage is limited in that the compressed air in the compressor stage is unable to follow the compressor blade contour arbitrarily, but rather separates, starting from the trailing edge of the compressor blade. The maximum stage pressure ratio rises as the mass flow rate increases, constituting the absolute limit of the stable operating range as the surge limit. The maximum mass flow rate of the compressor stage is limited by a stopper limit as soon as a velocity of flow corresponding to the velocity of sound forms in a flow cross-section, typically at the compressor entrance, and thereby limits the implemented mass flow rate.
Depending on the angle of attack or on the inflow velocity of the air onto the compressor blade, there is a difference in pressure between the upper face and the lower face. Since a compressor blade is a resilient component, it will yield to the difference in pressure between the upper face and the lower face and sag. As the load increases, the sag and thus the deflection of the compressor blades become more marked.
The operating range of the compressor is thus limited by the surge limit and by the stopper limit. In this case, the surge limit is an unfavourable, unstable operating state for the compressor, which state can lead to the destruction of the compressor. Especially when the compressor is used in reaction engines, it is absolutely essential to avoid this unstable operating state, in order to ensure operational reliability.
Surges arise if the mass flow rate required for a pressure ratio across a compressor stage is too low, or the pressure ratio for a specific mass flow rate is too large, thereby causing backflow and thus a stall. In this manner, the pressure ratio and the mass flow rate are altered momentarily, as a result of which the operating point is momentarily in the stable range and thereafter the unstable operating point in turn arises. This cyclical switching between the stable and the unstable operating state close to the surge limit may occur for example only at some compressor blades, only individual compressor blades experiencing a stall and this effect continuing counter to the direction of rotation of the compressor impeller. The cyclical switching of the flow causes cyclically alternating loads on individual compressor blades. Owing to the increasing stall and the alternate loading associated therewith, the compressor blade starts to vibrate, the compressor blades sagging as a result of this alternate loading and possibly breaking. However, if an unstable operating state exceeding the surge limit arises, this causes, however, a complete stall and considerable pressure surges in the compressor. In the power plant as a whole, this state poses a considerable danger on account of extinguishing flames, burning fuel in the compressor, overheating, deformations, etc., the compressor and thus the reaction engine possibly being completely destroyed.
The progression of the surge limit of the operating range is subject to operationally-induced and age-induced changes. The surge limit is thus influenced by changes in the environmental conditions during flight, inflow conditions of the compressor, by the thermal inertia of the components and by the penetration of foreign objects. Changes in the tip clearance of the compressor blades with respect to the housing, changes in the bearing play due to aging and wear, deformations and fouling of the blade geometries and at the housing also influence the surge limit.
A sufficiently large pressure ratio margin of the permitted operating states of the compressor with respect to the surge limit should allow for the reduction in the surge limit to lower pressure ratios which is triggered by these influences. The critical operating state is reached when the compressor accelerates, in the case of which compressor the surge limit margin is provisionally lowered. In practice, the surge limit margin for new power plants is set to be approximately 25% of the pressure ratio, in such a way that, by the end of the service life of the compressor, it has fallen to 5% owing to the age-induced reduction in the surge limit.
The optimum efficiency of a compressor is generally close to the surge limit, in the stable operating range, and this gives rise to a disadvantage of use due to the safety-relevant setting of the surge limit margin. Therefore, the prior art discloses devices and methods intended for operating compressors and for protecting the compressors from this dangerous operating state at optimum compressor efficiency. For example, blow-off valves are used to reduce the pressure ratio across a compressor stage. In many cases, an adjustment of rotatably mounted guide blades is provided, with which guide blades the pressure ratio or mass flow rate can be varied in order to thus ensure a reliable, stable operating state. Furthermore, actively changing the tip clearance of the compressor blade through heating or cooling the compressor housing is known. As a pre-condition in this regard, it is, however, necessary to reliably detect the operating state of the compressor and, accordingly, the surge margin of the current operating point of the compressor with respect to the surge limit.
The deflection of the compressor blade tip can be calculated from the temporal difference of the measured passing time of the compressor blade tip at one sensor at the housing and an ideal passage time that would occur with a compressor blade of ideal rigidity, and from the known tangential velocity of the compressor blade tip. “Passage time” shall be understood to mean the time at which the compressor blade tip is located, at least in part, in the sensor region of the sensor at the housing of the compressor. In this case, for example, entry into the sensor region, passage through the sensor region or exit from the sensor region may be defined in order to define the passage time.
U.S. Pat. No. 6,474,935 B1 describes the detection of rotating separations on the basis of the measurement of the deflection of the compressor blade tips due to pressure fluctuations caused by the rotating stall cell.
The method thus relates to the identification of precursors of an unstable compressor state. It is known that these precursors appear a few milliseconds prior to the onset of compressor instability, meaning that insufficient time now remains for performing counter measures, for example reducing the fuel mass, opening the blow-off valves or adjusting the guide blades.
DE 10 2008 036 305 A1 describes a method in which input power of the compressor is determined from the passage times of individual compressor blades. For this purpose, the real passage times are compared with the ideal model passage times, and the difference therebetween is evaluated as a consequence of the compressor blades having sagged. It is possible to calculate a compressor moment from the sag of the compressor blades and, accordingly, to calculate a compressor power using the rotational speed of the compressor. In the stable operating state, there is an equilibrium between the driving power and the compressor power. Any upset to the equilibrium of power is seen as oncoming instability and it is indicated that the surge limit is being approached.
The state, such as wear, soiling, erosion and deformations at the compressor blades, and changes in the position of the compressor blades, which re-orientate themselves by means of the blade base play upon each power plant start-up, influence the measured passage time with respect to the nominal passage time, on the basis of which the sag or deflection of the compressor blades is determined and a conclusion is drawn as regards the operating point and the surge margin thereof with respect to the surge limit. The methods known in the prior art are unable to detect and eliminate this influence, meaning that faulty detections may arise. It is not possible to reliably determine the operating point of the compressor and thus the surge limit margin of the operating point.